"'In other words, one partner stays 'on guard' while the other feeds -- these fishes literally watch each others' back," Dr Brandl says.

"'This behaviour is so far unique among fishes and appears to be based on reciprocal cooperation between pair members."

"Reciprocal cooperation, which requires an investment in a partner, which is later reciprocated, is assumed to require complex cognitive and social skills. Skills that fishes have been deemed not to have.

"Yet, Dr Brandl says their research shows clear coordination and presents intriguing evidence for reciprocal cooperation between the rabbitfish pairs.

"'There has been a long standing debate about whether reciprocal cooperation can exit in animals that lack the highly developed cognitive and social skills found in humans and a few species of birds and primates." Dr Brandl says.

"'By showing that fishes, which are commonly considered to be cold, unsocial, and unintelligent, are capable of negotiating reciprocal cooperative systems, we provide evidence that cooperation may not be as exclusive as previously assumed.'"